1. Field of the Invention
The present invention relates to a DTMF (Dual Tone Multi Frequency) signal processing method, a DTMF signal processing apparatus, a repeater, and a communications terminal apparatus, and more particularly, to a DTMF signal processing method, a DTMF signal processing apparatus, a repeater, and a communications terminal apparatus which erase a DTMF signal in a bidirectional communication of media data.
2. Description of the Related Art
In recent years, remarkable developments have been attained in VoIP (Voice Over Internet Protocol) which encodes speech data, and arranges the speech encoded data into packets for communication through networks. Also, increasingly more services have been provided for bidirectionally communicating a plurality of media data such as video data, character data and the like as well as speech data. The minimum configuration of a communications system for providing a bidirectional communication service comprises two communications terminal apparatuses between which bidirectional communications are made. However, standardization has not been fully accomplished in communications networks which are disposed between communications terminal apparatuses, call control system, media data encoding system, and communications system.
Examples of bidirectional communication services may include a multi-point teleconference service which permits multiple persons to make bidirectional communications among them, a call service for placing a call between a mobile terminal and an IP terminal. A communications system which provides such a bidirectional communication service realizes bidirectional communications of media data by interposing one or a plurality of repeaters between communications terminal apparatuses.
When a repeater is interposed as mentioned above, a communications terminal apparatus may request the repeater to switch from one service to another. This switching can be made by sending a DTMF signal as speech data, and by detecting the DTMF signal in the repeater, and switching the service, other than by using a call control message. In particular, a mobile terminal is highly likely to utilize the DTMF signal for switching in view of its configuration.
In a communications system as described above, even though the DTMF signal transmitted as speech data is information necessary for the repeater in order to switch services, the DTMF signal is unnecessary data for other repeaters which are installed beyond the repeater or for communications terminal apparatuses.
For this reason, repeaters and destination communications terminal apparatuses are expected to implement steps to erase the DTMF signal. For example, the repeater can replace speech data with either mute data or noise data in a section of the DTMF signal, again encode the resultant speech data, and send the speech encoded data in packets, or can omit transmission of packets in the section of the DTMF signal. The destination communications terminal apparatus in turn can replace speech data in a section of the DTMF signal with either mute data or noise data, and reproduce the resultant speech data to permit the user to hear the speech data, in a manner similar to that of the repeater.
As a related technology of this type, JP-A-2000-188611 (hereinafter called “Document 1”) discloses a communications apparatus which transfers a DTMF signal from a call communication path to a destination as data, and prevents double reception of the DTMF signal at the destination. Also, techniques for detecting the DTMF signal are described in JP-A-1997-327047 and JP-A-1993-207523 (hereinafter called “Document 2” and “Document 3,” respectively).
To erase speech data in a section of a DTMF signal, speech encoded data must be decoded, the decoded speech data must be accumulated for a certain section, and a determination must be made as to whether or not the speech data in the section is a DTMF signal in order to prevent an erroneous erasure. For this reason, an increased delay in speech is inevitable between communications terminal apparatuses, as compared with the case where the DTMF signal is not erased. Moreover, considering that the above-mentioned operations are performed during bidirectional communication in a communications system which comprises one or a plurality of intervening repeaters, the increased delay in speech will significantly impede the serviceability.
Also, in determining whether or not a DTMF signal is present, factors such as whether some packets have been lost in a packet network or the like, or whether data has been lost or corrupted due to radio errors or the like on a mobile network, must be also taken into account.